AMC 12 2003a Problem 4

It takes Mary $30$ minutes to walk uphill $1$ km from her home to school, but it takes her only $10$ minutes to walk from school to her home along the same route. What is her average speed, in km/hr, for the round trip?

$\mathrm{(A) \ } 3 \mathrm{(B) \ } 3.125 \mathrm{(C) \ } 3.5 \mathrm{(D) \ } 4 \mathrm{(E) \ } 4.5$